A crystal oscillator is a circuit that uses the mechanical resonance of a vibrating crystal to create an electrical signal with a precise frequency. This frequency is commonly used to keep track of time as in a quartz clock, to provide a stable clock signal for integrated circuits, and to stabilize frequencies for radio transmitters and radio receivers. The most common type of piezoelectric material used in a crystal oscillator is a quartz crystal, so oscillator circuits designed around quartz crystals are typically referred to as “crystal oscillators.” A quartz crystal provides frequency control by acting as a series resonant circuit. A parallel resonance associated with the crystal electrode capacitance limits how high in frequency the crystal may be operated. The series resonance is typically a few kilohertz lower than the parallel one. Crystals below 30 MHz are generally operated above series resonance and limited by the parallel resonance, which means the crystal appears as an inductive reactance in operation. Any additional circuit capacitance will thus pull the frequency down. For a parallel resonance crystal to operate at its specified frequency, the surrounding electronic circuit has to provide a total parallel capacitance as specified by the crystal manufacturer. Crystals above 30 MHz (up to >200 MHz) are generally operated at series resonance where the impedance appears at its minimum and is equal to the series resistance. For these crystals, the series resistance is specified (<100Ω) instead of the parallel capacitance. To reach higher frequencies, a crystal can be made to vibrate at one of its overtone modes, which typically occur near integer multiples of the crystal's fundamental resonant frequency. Only odd numbered overtones have resonant modes. Such a crystal is typically referred to as a 3rd, 5th, or even 7th overtone crystal. To accomplish overtone operation, the oscillator circuit typically includes an additional LC circuit to select the desired overtone.
Some examples of overtone crystal oscillators are disclosed in U.S. Pat. No. 4,709,218 to Moritani et al., entitled “Overtone Crystal Oscillator” and in U.S. Pat. No. 5,081,430 to Kohsiek, entitled “Overtone Crystal Oscillator Having Resonance Amplifier in Feedback Path.” Additional examples of overtone crystal oscillators utilize circuitry to provide resonant operation in a third overtone mode. Some of these examples are disclosed in U.S. Pat. No. 5,113,153 to Soyuer, entitled “High-Frequency Monolithic Oscillator Structure for Third-Overtone Crystals,” U.S. Pat. No. 7,332,977 to Drakhlis et al., entitled “Crystal Clock Generator Operating at Third Overtone of Crystal's Fundamental Frequency,” and U.S. Pat. No. 7,852,167 to Makuta et al., entitled “Third Overtone Crystal Oscillator.”